1. Field of the Invention
The invention relates to the field of power combining using cavity resonant devices. In particular the invention relates to the field of power combining using a resonant cavity which resonates in a higher order mode. More specifically the invention relates to a power combiner which utilizes two higher order mode resonant cavities neither of which requires mode suppressors thus providing a highly efficient device.
2. Prior Art
The most pertinent prior art known to me is that of three earlier patents granted to me and assigned to Motorola, Inc. of Schaumburg, Ill., the assignee herein.
In U.S. Pat. No. 4,034,314, I disclosed a microwave diode coaxial circuit oscillator which comprised an accumulating cavity, resonant in the dominant mode at the frequency f.sub.o, and loaded by a centered output probe. An unloaded cavity, dominant mode resonant at f.sub.o, was spaced a quarter wavelength from the accumulating cavity so as to appear as a virtual short circuit to f.sub.o energy which might not otherwise couple to the accumulating cavity and be lost in a terminating, stablizing load on the input coaxial line. Also disclosed was the use of an unloaded cavity, dominant mode resonant at twice f.sub.o, spaced an integral number of quarter wavelengths from an active energy device (a negative resistance diode) at its resonant frequency of twice f.sub.o. This second unloaded cavity provided a virtual short circuit or virtual open circuit to 2f.sub.o energy at the active device to control loading at the second harmonic of f.sub.o and reduce noise output from the accumulator cavity. The manner in which a multiplicity of coaxial input circuits might be coupled to the accumulator cavity for purposes of combining power output of several active devices was also disclosed. The number of such coaxial input circuits which could be utilized with the accumulator cavity was obviously limited by the inherent size of a cavity resonator resonant in the dominant mode at f.sub.o.
In U.S. Pat. No. 4,075,578, I introduced the concept of non-symmetrical input and output couplings to a dominant mode accumulator cavity to increase the number of energy circuits which might be coupled into and out of an existing dominant mode accumulator cavity. By locating output coupling devices at or near the cavity wall of the accumulator cavity much of the coupling mechanism may be located outside of the projection of the accumulator cavity wall, resulting in greater space availability within the cavity perimeter for additional coupling devices. As in my earlier patent, I provided a second non-loaded, dominant mode cavity, resonant at f.sub.o, to stabilize operation and to reduce losses.
To further improve space availability for coupling device I introduced in U.S. Pat. No. 4,143,334 the use of a cavity enlarged to resonate at f.sub.o in a higher order mode. Because the accumulator cavity was enlarged to accommodate the higher order mode of resonance space was made available for more coupling devices. Once again I utilized a second cavity which was also resonant at f.sub.o. However, unlike the accumulator cavity this second cavity was resonant in the dominant mode. As with my earlier teachings this second cavity was unloaded and spaced a quarter wavelength from the accumulator cavity to provide spurious-free and efficient operation.
A negative-resistance oscillator is an oscillator produced by connecting a parallel-tuned resonant circuit to a two-terminal negative-resistance device; that is, a device in which an increase in voltage results in a decrease in current. As a power source, the negative resistance device leaves much to be desired. In order to achieve reasonable power output levels from a negative resistance oscillator a plurality of such negative resistance active devices must be coupled to the resonant cavity.
Negative resistance devices, such as impact avalanche transit time (IMPATT) diodes are rich in frequency content by virtue of the large frequency bandwidth over which negative resistance can be obtained. While utilizing higher order mode resonant cavities, rather than dominant mode resonant cavities, provides a larger cavity with a greater surface area in which coupling devices may be incorporated, enlarging the cavity to permit higher order mode resonance in a negative resistance oscillator sets the stage for several resonances at frequencies other than the desired frequency of operation. Since the negative resistance energy source can output energy in many of these higher order mode resonant frequencies, the efficiency of the negative resistance oscillator at the desired frequency may be reduced if such extraneous resonances are permitted to be sustained.
The well known and well observed prior art teaching with respect to negative resistance oscillators utilizing dominant mode resonators was that the real part of the cavity input impedance should be maximized and the coaxial input circuit matched to this maximized cavity impedance. This would assure maximum gain at the frequency at which the real part of the cavity input impedance was maximized. For reference see N. D. Kenyon, "A Circuit Design For MM-Wave IMPATT Oscillators", 1970, G-MTT Symposium Digest, pages 300-303.
Unfortunately, when the prior art teaching is attempted with higher order mode resonators it is found that the real part of the input impedance to the cavity is maximized at several higher order mode resonant frequencies. In fact the input impedance at the desired modal resonant frequency is lower (due to output coupling) than the input impedance presented at modal resonant frequencies which it is desired to suppress.
The prior art has recognized this problem and after maximizing the real part of the cavity input impedance at the desired modal resonant frequency, designers have provided mode suppression means within the cavity to reduce the output of undesired modal resonant frequencies. However, this results in the loss of the energy contained in such suppressed modal resonant frequencies. Further, the Q of the cavity is adversely affected to the further detriment of oscillator efficiency.
It is therefore an objective of the invention to provide an efficient higher order mode, negative resistance oscillator and power combiner in which unwanted modal resonant frequencies are suppressed without detriment to the overall efficiency of the oscillator.
It is further objective of the invention to teach a new method for optimizing the real part of the input impedance of the higher order mode cavity resonator which will permit the negative resistance, active device to differentiate the desired modal resonant frequency from those which are undesired.
It is a specific objective of the invention to provide means and method for suppressing undesired modal resonant frequencies in an essentially loss-less manner.